1. Field of the Invention
The present invention relates to a radio communication apparatus such as a mobile phone (cellular phone) and particularly, to transmission power control for the radio communication apparatus.
2. Description of the Related Art
In a Code Division Multiple Access (CDMA) mobile communication system, a transmission power from a radio communication apparatus such as a mobile phone is variably controlled. That is, a transmission power from a plurality of mobile stations is controlled such that a base station can receive a signal that has been transmitted from the mobile stations at a constant power. This reduces interference between communication channels due to effects of near-far problem in a communication system using a plurality of mobile stations and allows a frequency resource to be effectively used.
To this end, in the CDMA mobile communication system, the transmission power control needs to be strictly performed. In a wide-area CDMA (W-CDMA) mobile communication system that has been developed by a 3GPP (3rd Generation Partnership Project), the maximum allowance and accuracy of the transmission power are specified. In this W-CDMA mobile communication system, the transmission power needs to be controlled in high accuracy by a small step size (1 dB, 2 dB or 3 dB) over a wide power range.
In transmission power control (TPC) in the W-CDMA mobile communication system, two TPC algorisms (TPC algorism is also referred to as “Power Control Algorism (PCA)”) are used. In “Algorithm 1”, a transmission power is controlled in every one slot. In “Algorithm 2”, a transmission power is controlled in every five slots. “Slot” is a unit of time direction and one slot corresponds to 0.6666 μsec (=2560 chip (1 chip=1/3.84 MHz)). Accordingly, transmission power controls are performed 1500 times per second in “Algorithm 1” and 300 times per second in “Algorithm 2”. Further, in “Algorithm 1”, power control of 1 dB or 2 dB is performed at one time (per one slot). In “Algorithm 2”, power control of 1 dB is performed at one time (per five slots).
However, the linearity of input power versus output power of a power amplifier (hereinafter, referred to as merely “PA”) used in the TPC decreases in a high output state due to a variety of factors, so that the absolute accuracy of the transmission power decreases in general as the transmission power becomes higher. In addition to such a behavior in a saturation region of the PA, the transmission power may not be controlled to a desired value in some cases in a radio circuit of a mobile phone due to an error in values of an analog element (analog error), temperature characteristics of respective devices, an error (ripple) in frequency characteristics of a filter, and the like.
To improve the above disadvantage, various methods have been proposed. One of them is a method that compensates (corrects) a transmission power by feedback control to increase the accuracy of the control (refer to, for example, JP-A Nos. 11-308126, 2001-223637, and 2004-208180).
As a feedback system for increasing the accuracy of power control, one makes the transmission power that has been actually supplied and transmission power that has been set by the TPC equal to each other. This system is called Automatic Power Control (APC) or Automatic Level Control (ALC) and compares a measurement value of the power that has been actually transmitted and a value that has been set by the TPC to obtain an error amount between them and feeds back the error amount for adjustment.
Further, in order to compensate an error at the portion where the setting value of the transmission power is high (in the vicinity of the maximum transmission power) in particular, a mechanism in which the APC becomes effective when the transmission power becomes higher than a predetermined threshold to allow compensation of the transmission power to operate is adopted in general. This is because a power range that a widely-used diode detector can detect is restricted. If the power range is extended, a circuit configuration becomes complicated to increase cost.
FIG. 1 is a graph showing an example of a normal relation between a setting value in the TPC and transmission power. In FIG. 1, line 1a represents an ideal transmission power. Dotted line represents a threshold value for making the APC effective (ON). When the transmission power becomes higher than the threshold value, the APC becomes effective. Curved lines 1b and 1c represent a transmission power that has not been compensated by the APC. The portion where the transmission power is less than the threshold value, that is, where the APC is not effective is the curved line 1b. The portion where the transmission power is higher than the threshold value, that is, where the APC should be effective is the curved line 1c. 
The actual transmission power does not completely correspond to the ideal line 1a due to an analog error and the like. Although the transmission power deviates from the line 1a in the range where the transmission power is low, influence of the error is small. Therefore, the deviation is still within an allowable range even if the APC is not effective. In FIG. 1, although the curved line 1b deviates from the line 1a due to an analog error and the like, the deviation is relatively small.
On the other hand, influence of the analog error and the like is large in the range where the transmission power is high, so that the APC needs to be effective. In FIG. 1, the curved line 1c deviates greatly from the line 1a due to various errors in analog devices (temperature characteristics, frequency characteristics, adjustment error, etc.). Further, in the case of a transmission power setting value for the non-linear area of the PA, an error is not a quantitative value, so that a simple compensation processing is not enough. In this case, therefore, it is necessary to extend the threshold value and compensate the transmission power by means of the APC.
In the APC, the control of the transmission power is so performed as to cancel the error. That is, a compensation amount that brings the curved line 1c close to the line 1a is added.
The curved lines 1b and 1c in FIG. 1 are lower than the line 1a. However, the curved lines 1b and 1c may become higher than the line 1a in some cases.
Conventionally, in order to bring the transmission power close to a desired value as quickly as possible, the compensation of the transmission power by the APC has been performed in every one slot. In this case, the waveform of the transmission power that has been compensated by the APC does not correspond to the waveform to be obtained in the TPC as a transitional behavior in terms of temporal change.
In a radio communication apparatus provided with a conventional APC function, in order to bring an actual transmission power close to a target transmission power as quickly as possible, the control of the transmission power is performed in every one slot time which is a base unit irrespective of timing that has been specified by a higher layer or algorithm. Therefore, even in the case where an instruction to perform the transmission control in every five slots according to algorithm 2 in the TPC is issued, the control may be performed in every one slot in the APC in some cases. As a result, the obtained waveform of the transmission power considerably differs from a waveform desired in the TPC. That is, in the case where an error amount between the ideal transmission power and actual transmission power is large, a transmission power waveform to be obtained in the APC cannot be obtained at the boundary between ON zone and OFF zone due to influence of response time (time needed to reach convergence) in the APC.
In the conventional radio communication apparatus, the transmission power is increased or decreased by 1 dB in each 5-slot cycle at the portion other than the boundary. At the boundary portion, the control of the transmission power is not performed according to the regular operation. 3GPP specifies that the transmission power should not be changed during TPC control cycle as a general rule. In this regard, since a given error is allowed for the transmission power in 3GPP, it is not always true that the conventional operation deviates from the specification in dependence on a change in the amount of the power. However, it can be said at least that a power change different from that the base station assumes in the TPC is shown, so that there is a possibility that the conventional operation has adversely affected the interference between channels and frequency use efficiency.